Two things to try:
1) Select a group of portrait and Landscape images exhibiting the issue and then go to Library> Previews> Render 1:1 previews with all of the images selected.
Try browsing these images after all previews are rendered and see if still blurry. If so try #2:
2) Update your graphics card driver using the latest version downloaded from the manufacturer's website (AMD, NVIDIA). There is usually no need to uninstall the previous driver, simply load the update. You can always rollback the driver.